The invention relates to a torque sensing calibration unit for calibrating or confirming the accuracy of calibration of torque sensing equipment such as a calibrated torque wrench or a torque sensor.
All measuring equipment if certified to operate to a particular degree of accuracy must be true to that standard of accuracy both immediately after manufacture and initial sale and after prolonged use. It is therefore necessary for all measuring or sensing equipment which carries such a certificate of accuracy to be regularly checked to confirm that the accuracy is maintained. Alternatively the equipment must be regularly recalibrated to the stated degree of accuracy. In the case of torque sensing equipment, such as torque indicating torque wrenches, frequent recalibrations are desirable. A typical calibration standard for a torque wrench would be a long arm beam designed to resist applied torque by means of a reaction torque generated by a proof mass suspended in a scale pan a significant distance, probably more than 2 meters, from a fulcrum of the beam. Torque is applied to the beam about an axis that coincides with the pivotal axis of the beam, and beam movement is prevented by the careful addition of proof masses to the scale pan at one end of the long arm beam until a balancing reaction torque is obtained. The entire calibration must be carried out in a temperature-controlled room, in conditions of controlled humidity, the room having a stable floor that is substantially free of vibration. The recalibration process is slow, as it involves the addition of proof masses individually to the scale pan. The recalibration can only be carried out at a certified testing site where the massive calibration equipment is permanently installed and where the force of gravity, g, is accurately known. The reaction torque xcfx84 applied by the long arm beam can however be derived directly as the product
xcfx84=dxc3x97Mxc3x97g
where
d is the distance from the fulcrum of the beam to the scale pan,
M is the total proof mass added to the scale pan to prevent angular movement of the beam in response to an applied torque, and
g is the acceleration due to gravity at the test site.
That measured reaction torque can be compared with the corresponding value as indicated by the torque sensor on the torque wrench, as a direct check on the accuracy of calibration of the sensor.
New proposals for desirable standards of calibration equipment and calibration procedures have suggested that any single calibration or recalibration of a torque measuring instrument should check the accuracy of the instrument""s calibration in a series of measurements spanning a torque range of from 10% of the maximum rated torque of the instrument being calibrated to about 100%, in 10% increments. Furthermore, it has been suggested that each individual measurement should be repeated a number of times, with an average or a statistically weighted average being used to indicate the reaction torque being applied by the calibration equipment. It is impractical to use existing long arm beam calibration equipment in such repetitive calibration procedures, and there has emerged a need for an automatic machine capable of making a series of torque measurements as a predetermined automatically run cycle, each measurement being made to a certified degree of accuracy. Furthermore that accuracy needs to be at least as good as that of existing long arm beam calibration equipment. Preferably such automatic equipment should be able to permit automatic repeat measurements so that each torque measurement is taken a number of times to check and confirm its accuracy, and should be capable of automatically scanning across a defined range of torque loads, in a series of defined increments. Other desiderata attaching to the design of any such calibration equipment are that it should preferably be portable, should not require a knowledge of the acceleration due to gravity, g, at the test site, and that it should be capable of recognizing and allowing for the existence of side loads.
The invention is based on the realization that a secondary measuring instrument may be acceptable as a definitive reference for torque calibration if the accuracy of that secondary instrument is significantly greater than that of the primary torque reference measurement calibration apparatus.
The invention provides a torque sensing calibration unit comprising a hollow torsion bar to which torque is to be applied, associated with at least two pairs of position sensors, each pair being arranged to measure movement of the bar on diametrically opposite sides of a central axis through the bar, and the pairs of sensors being arranged to measure movement in mutually perpendicular directions.
Preferably the hollow torsion bar has a disc-shaped terminal end and a first one of the two or more pairs of sensors is arranged to detect rotary movement about the central axis of the torsion bar. That can be achieved by using the first pair of position sensors to monitor the positions of a pair of flat coplanar faces of side lugs of the disc-shaped terminal end, the plane of the flat faces of the side lugs being coincident with the central axis of the torsion bar. The lugs are preferably equidistant from the central axis, so that pure rotations of the hollow torsion bar would move the lugs in equal and opposite directions. A side load applied to the torsion bar in a direction perpendicular to the plane of the flat faces would however cause deflection of the hollow torsion bar so as to move the lugs in the same direction, so that an analysis of the lug movements relative to one another gives accurate and independent identifications of both rotary movement and linear transverse movement of the torsion bar in the direction of measurement due to side loads.
A second pair of the position sensors is preferably arranged to detect rotary movement about an axis transverse to the central axis of the torsion bar, by monitoring the positions of a pair of target zones on the face of the disc-shaped terminal end of the torsion bar or on the side lugs, on diametrically opposite sides of the central axis. Analysis of the sensor outputs of the second pair gives an accurate identification of linear movement of the torsion bar due to side loads in a direction perpendicular to the side loads producing the transverse movement measured by the first pair of position sensors.
The third pair of the position sensors is arranged similarly to the second pair, but in a mutually perpendicular plane.
As an alternative to the second and third pairs of position sensors described above, a second pair of the position sensors may be arranged to monitor the positions of a pair of flat coplanar faces of a second pair of side lugs of the disc-shaped terminal end, the plane of the flat faces of the second pair of side lugs being coincident with the central axis of the torsion bar and perpendicular to that of the flat faces of the first pair of side lugs. In such an arrangement the analysis of the movement of the second pair of lugs relative to one another gives an accurate measure of rotary torsion induced movement and of linear side load induced movement of the torsion bar, independently of one another. Together, the first and second pairs of sensors therefore measure, independently, movement due to torque and movement due to side loads in two mutually perpendicular directions. A third pair of sensors could if appropriate monitor movement due to side loads along the third axis.
For ease of processing the data from the two or more pairs of sensors, all preferably monitor movement of areas of the terminal end of the torsion bar at equal distances from the central axis. Preferably the second and third pairs are arranged in a horizontal plane and in a vertical plane respectively through the central axis.
Because the analysis of data from the position sensors includes data identifying torque (rotational) loads and data identifying side loads in each of two or three three mutually perpendicular axes, it is possible when processing data from the calibration unit according to the invention to present results which represent pure torque as applied to the central axis of the torsion bar, as well as independently measured side load data.
Preferably the hollow torsion bar and terminal disc portion are machined from a single piece of high quality stainless steel, with all six position sensors monitoring movement of flat faces of the disc portion or associated with the disc portion spaced from the central axis by the same radial distance which may for example be from 10 to 20 cms.
The position sensors may be optical sensors which are capable of sensing to an accuracy of 1 part in 100,000. Because of the orientation of the three pairs of sensors, the equipment can sense torsional twisting of the hollow torsion bar as well as net vertical and net lateral movement of the distal end of the torsion, bar, being the end with which the sensors are associated.
In use, if a torque load is applied to the hollow torsion bar, very slight rotary flexure of the torsion bar is invariably likely to be produced and is detected by the two or more pairs of position sensors. A comparison of the measurements of the position sensors of each pair will indicate whether there is vertical or lateral movement of the hollow torsion bar under an applied side load, as well as rotary twisting of the arm under the applied torque load. Preferably the calibration unit is associated with a computer which is capable of processing the different position sensor readings from each applied torque, to provide a final torque measurement output figure alongside figures representing side loads in the vertical and horizontal planes. The signal processing is described in greater detail below in conjunction with the illustrated embodiment which is given by way of example.
The calibration unit of the invention is not an absolute measurement of torque in the same way as a long arm beam is. It must itself be calibrated, but since it is capable of torque measurements to an accuracy several orders of magnitude higher than that of the long arm beam which is the primary conventional calibration unit, and since it is capable of retention of its calibrated accuracy over long periods of use, the unit is perfectly acceptable as a secondary calibration unit. Preferably the unit is designed as a portable unit contained in a housing which has a constant temperature and humidity.
For calibration purposes, the unit of the invention is provided with a mounting frame which has two mutually perpendicular support surfaces, and a levelling means to enable the unit to be positioned on a flat surface with one of those support surfaces precisely horizontal. The unit is calibrated at a certified testing site where the acceleration due to gravity, g, is accurately known. After levelling, a scale pan is attached to the end of the torsion bar, and proof masses added to the scale pan to apply a precisely known torque and side load to the bar. The measured torque and side load from the unit can be compared with that applied, and the measurements repeated for a statistically meaningful number of measurements with the same or different applied torques and/or side loads. Then the unit is rotated through 90xc2x0 until it is positioned on the same flat surface but resting on its other support surface. The measurements of applied torque are repeated, but because of the rotation of the unit the side load element of the first series of measurements are absent, as far as that pair of sensors is concerned in the final calibration. Thereafter the unit, without the scale pan and proof masses, can be carried to any factory site and used to calibrate torque sensing tools, since the deflection of the torsion bar due to applied torque is not dependent on the acceleration due to gravity, g, at the factory site.